"Relativity will try to realize its more ambitious long-term goal: taking this manufacturing process to the Red Planet and building a rocket using 3D printers on Mars."
Why not aim to build a fuel factory first, to refuel spacecraft for return trips?
Maybe because a fuel factory wouldn't be immediately profitable on Earth like building rockets? On Earth there are plenty of cheaper ways to make fuel.
I do agree though that fuel is the most important thing to start making on Mars though. That and air and water.
Rocket Lab is also using 3D printing successfully on their process: https://www.rocketlabusa.com/